Many vehicles utilize catalysts in exhaust systems to reduce emission. In lean exhaust conditions, such as with regard to diesel exhaust or other lean burning conditions, a catalyst may utilize reductant other than burnt fuel. One such aftertreatment device is a Selective Catalytic Reduction (SCR) system, which uses a catalyst to convert NOx to nitrogen and water. A urea-based SCR catalyst may use gaseous ammonia as the active NOx reducing agent, in which case an aqueous solution of urea may be carried on board of a vehicle, and an injection system may be used to supply it into the exhaust gas stream.
At ambient temperatures of less than −11° C., the aqueous urea solution (comprising 32.5% urea and 67.5% water) may freeze in the on board urea storage tank. Thus, a pick-up tube of the injection system may not be able to deliver urea to the injector for delivery to exhaust gas and NOx reduction. In one approach, the urea storage tank includes an electric heating system to warm the frozen urea. Further, components of the urea storage tank and reductant injection system may have a freeze-safe design to assure functionality and survivability of the injection system over multiple freeze/thaw cycles.
The inventors of the present application have recognized a problem in the above solutions. First, there may be increased cost associated with the heating and freeze-safe components for the urea storage tank and reductant injection system. Second, fuel economy may be decreased by using energy produced by the vehicle to heat the entire urea tank, and such heating may take an extended duration, thus reducing the amount of exhaust gasses that can be treated catalytically with the reductant, and thus increasing exhaust emissions overall.
Accordingly, in one example, some of the above issues may be addressed by a liquid reductant injection system. The liquid reductant injection system includes a storage tank housing a reductant solution, a return conduit extending into the storage tank, the return conduit including an outlet positioned in the storage tank, and a thermosyphon comprising an evaporator coupled to an exhaust conduit and in fluidic communication with a condenser coupled to the return conduit inside the storage tank, the condenser positioned vertically above the evaporator.
In this way, waste heat from the exhaust system can be used to passively heat the reductant solution in the storage tank via the thermosyphon. In some examples, the thermosyphon is a closed-loop thermosyphon. Thus, in such an example, the thermosyphon does not need an outside power source or controller to operate although such components could be used, if desired. As a result, the reductant solution is heated without decreasing the fuel economy by using energy produced by combustion to heat the storage tank.
Additionally, in some examples the reductant solution may include ethanol. By including ethanol in the reductant solution, a freezing point temperature of the liquid reductant may be reduced. As such, the occurrences of reductant freezing may be reduced and/or the size and/or operating temperature range at which thermosyphon functions can be decreased.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.